1. Field of Invention
The present invention relates to a method for treating a surface of a wafer. More particularly, the present invention relates to a method for removing residues on the surface of the wafer.
2. Description of Related Art
In the semiconductor industry, the manufacturing of integrated circuits (ICs) can be categorized as three stages: wafer production, IC fabrication and IC packaging. In general, the wafer is fabricated into chips through layout design, lithography/etching and wafer scribing. After completing the integrated circuits of the wafer, a plurality of bonding pads are formed on the active surface of the wafer and a passivation layer is formed to cover the active surface. The bonding pads exposed by the passivation layer are electrically connected to the external medium, for example, a package substrate or a circuit board. Taking the flip chip package as an example, after forming the bonding pads, bumps are formed on the wafer surface to electrically and mechanically connect the external package substrate or the circuit board. After the formation of bumps, the wafer is scribed into individual chip packages. Since bumps have high reliability and are arranged on the bonding pads in arrays, they are suitable to be applied in high-density flip chip packages, including flip chip/ball grid array packages.
FIGS. 1A–1G are cross-sectional display views illustrating a conventional fabrication process for forming bumps. Referring to the FIG. 1A, a wafer 110 having an active surface 112 is provided. The wafer includes bonding pads 116 and a passivation layer 114 on the active surface 112. The bonding pads 116 are exposed through openings 118 of the passivation layer 114, so that the wafer can be electrically connected to the external circuit (not shown) through the exposed bond pads.
Referring to FIG. 1B, a metal layer 130 is formed over the active surface 112 of the wafer 110, for enhancing adhesion. The metal layer 130 is a composite layer formed by either sputtering or evaporation. The material of the metal layer 130 includes titanium, tungsten, chromium, nickel, copper and the alloys thereof.
After forming the metal layer 130 on the active surface 112 of the wafer 110, a plurality of bumps are formed on the bonding pads 116 by either electroplating or screen printing.
Taking electroplating as an example, the following steps are described in FIGS. 1C–1E.
As shown in FIG. 1C, a photoresist layer 140 is formed on the metal layer 130. After lithography and developing, the photoresist layer 140 is patterned to form a plurality of openings 142 therein, while openings 142 expose a portion of the metal layer 130.
Referring to FIGS. 1D–1E, the openings 142 are filled by solder paste to form a plurality of solder (bump) paste globs 150 within the openings 142 by electroplating. The location of each solder paste glob 150 (opening) corresponds to the location of each bonding pad 116. Afterwards, the photoresist layer 140 is removed, leaving a portion of metal layer 130 between the solder paste globs 150 being exposed.
Referring to FIGS. 1F–1G, using the solder paste globs as an etching mask, a wet etching process is performed to removed the exposed metal layer 130 between the solder paste globs 150, thus forming an under bump metallurgy (UBM) layer 132. Next, a reflow step is performed, so that the solder paste glob 150 becomes a globular bump 152.
However, as shown in FIG. 1E, if some photoresist 134 is not completely removed but remained on the metal layer 130 between the solder paste globs 150, the remained photoresist 134 will cause problems in the subsequent wet etching process. Because the remained photoresist 134 covers the underlying portion of the metal layer 130, the exposed metal layer 130 between the solder paste globs 150 can not be completely removed during the wet etching process and residual metal 136 stays on the passivation layer 114 between the globs 150. The residual metal can deteriorate the yield of the wafer. On the other hand, if the wet etching process is prolonged to remove the residual metal entirely, undercut of the UBM layer 132 may occur, which likewise decreases reliability for the flip-chip packages. Therefore, it is important to remove residues of the UBM layer without degrading the UBM layer.